1. Field of the Invention
The present invention relates to an organic light emitting display, and more particularly, to an organic light emitting display with a high image quality.
2. Discussion of the Related Art
An organic light emitting display is a self-luminous display that emits light by electrically exciting a fluorescent organic compound, and displays an image by driving N×M organic light emitting diodes (OLEDs).
There are two driving methods for the organic light emitting display, that is, a passive matrix (PM) method and an active matrix (AM) method. In the case of the PM method, anode electrodes and cathode electrodes are formed perpendicular to one another and the display is driven by selecting lines. In the case of the AM method, transistors and capacitors are connected to pixel electrodes and the display is driven to maintain voltages supplied from the transistors at the pixel electrodes using the capacitors.
FIG. 1 is a circuit diagram of one of N×M pixels in a related art AM organic light emitting display. Referring to FIG. 1, a unit pixel of the related art AM organic light emitting display includes a first transistor M1 (switching transistor), a second transistor M2 (driving transistor), a capacitor C1, and an OLED. Here, the first transistor M1 has a gate connected to a gate line 1, a source connected to a data line 2, and a drain connected to a node A. The second transistor M2 has a gate connected to the node A, and a source connected to a power supply line 3. The capacitor C1 is connected between the gate and source of the second transistor M2, and the OLED is connected to the drain of the second transistor M2.
The first transistor M1 is turned on by a selection signal Vs (or a scan signal) supplied through the gate line 1, and a data signal “Vdata” is supplied through the turned-on first transistor M1 to the node A. A voltage difference between both terminals of the capacitor C1 is a difference between a power voltage “VDD” and the data signal “Vdata”. In the second transistor M2, a driving current “IOLED ”of the OLED is determined according to the value of Vdata. The driving current “IOLED ”is expressed as Equation 1 below.IOLED=K(VDD−Vdata−|Vth|)2  (Equation 1)In Equation 1, “IOLED”, “K”, “VDD”, “Vdata”, and “Vth” represent a driving current of the OLED, a constant, a power voltage actually applied to the OLED, the data signal, and a threshold voltage of the second transistor M2, respectively.
The driving current IOLED of the OLED varies according to the data signal Vdata because the power voltage VDD and the threshold voltage Vth are generally constant. The luminance of light emitted from the OLED is determined according to the value of IOLED. Accordingly, a desired gray scale can be produced from the OLED by changing the value of the data signal Vdata.
Meanwhile, an organic light emitting display, as well as other flat panel displays, is also being actively researched to increase the size of its screen. The power voltage VDD is supplied to all the pixels of the display through the power supply line 3 which is aligned vertically in FIG. 1 (that is, from an upper side to a lower side). Generally, the power supply line 3 has an inherent line resistance. In the case of a wide-screen organic light emitting display, the power supply line 3 has an increased line resistance due to its increased length. In such a case, a considerably reduced power voltage VDD is actually applied to the pixels located at a lower side of the display due to a voltage drop (IR drop) caused by the increased line resistance, while the pixels located at a upper side of the display are supplied with a predetermined power voltage VDD.
As expressed by Equation 1, an image gray scale can be accurately produced by the data signal Vdata when a desired power voltage VDD is uniformly supplied to all the pixels (or all the active pixels) in the display. However, as described above, although a desired power voltage VDD is supplied to all the pixels, the actual power voltage VDD supplied to the pixels located at a lower side of the display becomes smaller compared with the actual power voltage supplied to the pixels located at a upper side of the display. Accordingly, gray scales produced by the lower pixels become lower than gray scales produced by the upper pixels, thereby causing nonuniformity in image quality.